A lean-burn engine may be supplied with a lean mixture of air and fuel (oxygen-rich mixture) as a means to improve vehicle fuel economy. The exhaust emitted from such engines during periods of lean-burn operation may include a relatively high content of oxygen (O2), a relatively low content of carbon monoxide (CO) and unburned/partially-burned hydrocarbons (hereafter HC's), possibly some suspended particulate matter, and small amounts of nitrogen oxides primarily comprised of NO and NO2 (collectively referred to as NOX gases). The NOX gas constituency of the exhaust may fluctuate between about 50 and 1500 ppm and may comprise greater than 90 wt. % NO and less than 10 wt. % NO2. The hot engine exhaust, which can reach temperatures of up to about 900° C., often needs to be treated to decrease the concentration of some or all of these gaseous emissions before it is expelled to the atmosphere from the vehicle's tailpipe.
To this end, an exhaust aftertreatment system may be installed downstream of the lean-burn engine to control the various unwanted emissions and particulate matter contained in the engine's exhaust. A typical exhaust aftertreatment system usually aspires to cooperatively (1) oxidize CO into carbon dioxide (CO2), (2) oxidize HC's into CO2 and water (H2O), (3) convert NOX gases into nitrogen (N2) and O2, and remove any other unwanted matter. Catalytic converters that employ platinum group metals (PGM's) largely comprised of platinum have long been used to address this need. But the nature of the exhaust produced during lean-burn engine operation poses certain challenges for traditional catalytic converters. One specific challenge is that the relatively high content of O2 and the relatively low content of CO and HC's in the hot exhaust renders the reaction kinetics for the conversion of NOX gases to N2 over PGM's quite unfavorable. The conversion of NOX gases into N2 may nonetheless be accomplished by several known approaches.
A lean NOX trap, or LNT, is but one available option that may be employed. A LNT generally operates by feeding the exhaust expelled from the lean-burn engine across and/or through an LNT catalyst material that exhibits NOX gas trapping and conversion capabilities. The LNT catalyst material generally includes an oxidation catalyst, a NOX storage catalyst, and a NOX reduction catalyst. When the lean-burn engine is combusting a lean mixture of air and fuel, the oxidation catalyst oxidizes NO to NO2 and the NOX storage catalyst traps or “stores” NO2 as a nitrate species. The oxidation catalyst may also oxidize other gaseous emissions contained in the engine exhaust such as CO and HC's, if present. The NOX storage capacity of the LNT catalyst material, however, is not unlimited and at some point may need to be regenerated or purged of the NOX-derived nitrate compounds. The LNT catalyst material may be regenerated, for example, by momentarily switching the mixture of air and fuel supplied to the lean-burn engine from lean to rich. The resultant delivery of rich-burn exhaust effluents to the LNT catalyst material causes the NOX-derived nitrate compounds to become thermodynamically unstable which, in turn, triggers the release of NOX gases and the regeneration future NOX storage sites. The liberated NOX gases are then reduced, largely to N2, by the excess reductants—such as CO, HC's and/or H2—present in the rich-burn engine effluents over the NOX reduction catalyst.
Another option that may be employed in the exhaust aftertreatment system to convert NOX gases to N2 is a urea/ammonia selective catalyst reduction system (urea-SCR). A urea-SCR system injects urea from an on-board and refillable urea storage tank into the exhaust expelled from the lean-burn engine. The urea decomposes in the oxygen-rich exhaust to form ammonia (NH3) which, in turn, selectively reduces the NOX gases to N2 and H2O in the presence of O2 over a catalyst material specific to that reaction. Ammonia may also be directly injected into the exhaust to accomplish the same result if desired.
A diesel oxidation catalyst (DOC) may be located upstream from the LNT or urea/ammonia-SCR system to oxidize as much of the NO (to NO2) and residual CO and HC's (to CO2 and H2O) contained in the engine's exhaust as possible. The initial oxidative impact of the DOC may be quite helpful since both LNT's and urea/ammonia systems generally convert NOX gases to N2 more efficiently as the ratio of NO to NO2 in the total NOX gas emission decreases to about 1:1 or lower. The early oxidation of CO and HC's may also help drive the conversion of NOX gases to N2 since the presence of these and other reductants can promote the partial decomposition of NO2 back into NO. The DOC may include a DOC catalyst material similar in composition to the LNT catalyst material except that it generally does not include a NOX storage or a NOX reduction catalyst.
Both the DOC catalyst material and the LNT catalyst material have conventionally been equipped with a refractory metal oxide on which the oxidation catalyst, generally platinum, is dispersed to oxidize NO, CO, and HC's. But the use of platinum in conventional DOCs and LNTs, especially the relatively large amounts that are normally employed to oxidize NO to NO2, is rather expensive. Platinum has also been shown, in some instances, to exhibit poor thermal durability and to lose some catalytic activity when exposed to engine exhaust at higher operating temperatures.
Conventional DOC and LNT catalyst materials, moreover, have demonstrated a general susceptibility to sulfur poisoning when the lean-burn engine combusts a sulfur-containing fuel such as a diesel fuel. The sulfur content of many diesel fuels is typically about 50 ppm or less and, for some ultra-low sulfur diesel fuels, about 10 ppm or less. This small amount of sulfur is oxidized mostly to SO2 when the sulfur-containing fuel is combusted. The SO2 may be further oxidized to SO3 when exposed to platinum or some other oxidation catalyst. The SO3 may then form particulates containing sulfuric acid when exposed to water vapor in the engine's exhaust. The SO2, SO3, and sulfuric acid-containing particulates may chemisorb as sulfur species including various sulfates and sulfites onto the platinum and other catalysts (PGM's, NOX storage catalysts, etc.) that may be contained in the DOC and the LNT catalyst materials. The relatively strong metal-sulfur bonds formed through such chemisorption enables the deposited sulfur species to block active catalytic sites and progressively diminish the catalytic conversion efficiency of the DOC and/or LNT. Several “deSOX” approaches have been developed that can help remove the deposited sulfur species from the DOC and/or LNT catalyst materials. But these approaches are often cumbersome and tend to penalize the fuel economy of the lean-burn engine.
The need for innovative developments that can help improve the operation and efficiency of exhaust aftertreatment systems for lean-burn engines, as well as other related fields of technological art, is thus prevalent and ongoing.